The sharpness of a video display may be improved by varying the horizontal scanning rate of the beam in response to variations of the luminance component of the video signal. The luminance signal is differentiated and the differentiated luminance signal is used to generate a current for driving an auxiliary beam deflection element, for example, a scan velocity modulation (SVM) coil to modify the horizontal scanning rate so as to emphasize contrast between light and dark areas of the display. For example, at a transition from black to white in a given horizontal scan line, the beam scanning speed is increased approaching the transition, thus making the display relatively darker in the black area of the transition. Upon passing the transition into the white area, the beam speed is decreased such that the beam dwells relatively longer, making the display relatively brighter. The reverse occurs in passing from light to dark.
The SVM coil operates to add or subtract from the magnetic horizontal beam deflection field applied by the main horizontal deflection coils. The angle of beam deflection is a function of horizontal rate scanning current, generally a sawtooth current. The horizontal rate scanning current causes the beam to sweep across a horizontal raster line at a vertical position determined by a vertical rate sawtooth current, coupled to the vertical deflection coils.
The sawtooth scanning drive currents are adjusted to account for the fact that the display screen is substantially flat rather than spherical. A given amount of angular beam deflection produces a smaller linear horizontal displacement of the beam at the center of the flat screen and a greater amount at the edges of the screen, because the screen is relatively farther from the source of the beam when scanning at the edges of the screen than at the center of the screen.